Horizontal gene transfer and microbial evolution: Is the Tree ...
Terminology Branches, splits, bipartitions In a rooted tree: clades (for urooted trees sometimes the term clann is used) Mono-, Para-, polyphyletic groups, cladists and a natural taxonomy The term cladogram refers to a strictly bifurcating diagram, where each clade is defined by a common ancestor that only gives rise to members of this clade. I.e., a clade is monophyletic (derived from one ancestor) as opposed to polyphyletic (derived from many ancestors). (Note: you do need to know where the root is!) A clade is recognized and defined by shared derived characters (= synapomorphies). Shared primitive characters (= sympleisiomorphies , aternativie spelling is symplesiomorphies) do not define a clade. (see in class example drawing ala Hennig). To use these terms you need to have polarized characters; for most molecular characters you don't know which state is primitive and which is derived (exceptions:....). What is in a tree? Trees form molecular data are usually calculated as unrooted trees (at least they should be - if they are not this is usually a mistake).
To root a tree you either can assume a molecular clock (substitutions occur at a constant rate, again this assumption is usually not warranted and needs to be tested), or you can use an outgroup (i.e. something that you know forms the deepest branch). For example, to root a phylogeny of birds, you could use the homologous characters from a reptile as outgroup; to find the root in a tree depicting the relations between different human mitochondria, you could use the mitochondria from chimpanzees or from Neanderthals as an outgroup; to root a phylogeny of alpha hemoglobins you could use a beta hemoglobin sequence, or a myoglobin sequence as outgroup. Trees have a branching pattern (also called the topology), and branch lengths. Often the branch lengths are ignored in depicting trees (these trees often are referred to as cladograms - note that cladograms should be considered rooted). You can swap branches attached to a node, and in an unrooted you can
depict the tree as rooted in any branch you like without changing the tree. Test:Which of these trees is different? More tests here C B A C B a C b a
c b a c b a I II III IV
V D -> d C -> c B -> b A -> a C B A C B a C
b a c b a c b a I II III
IV V D -> d Shared derived characters (aka synapomorphies) define monophyletic groups C -> c B -> b A -> a D
C B D C B D C b D c b d c
b I II III IV V D -> d Shared primitive characters (aka symplesiomorphies
) define paraphyletic groups C -> c B -> b A -> a D C B A D C B a
D C b a D c b a d c b a I
II III IV V D -> d Characters that are not shared, even if they are derived (as the autapomorphy d), do not provide phylogenetic information.
C -> c B -> b A -> a Terminology Related terms: autapomorphy = a derived character that is only present in one group; an autapomorphic character does not tell us anything about the relationship of the group that has this character ot other groups. homoplasy = a derived character that was derived twice independently (convergent evolution). Note that the characters in question might still be homologous (e.g. a position in a sequence alignment, frontlimbs turned into wings in birds and bats). paraphyletic = a taxonomic group that is defined by a common ancestor, however, the common ancestor of this group also has decendants that do not belong to this taxonomic group. Many systematists despise paraphyletic groups (and consider them to be polyphyletic). Examples for paraphyletic groups are reptiles and protists. Many consider the archaea to be paraphyletic as well.
holophyletic = same as above, but the common ancestor gave rise only to members of the group. The Coral of Life (Darwin) Coalescence the process of tracing lineages backwards in time to their common ancestors. Every two extant lineages coalesce to their most recent common ancestor. Eventually, all lineages coalesce to the cenancestor.
t/2 (Kingman, 1982) Illustration is from J. Felsenstein, Inferring Phylogenies, Sinauer, 2003 Coalescence of ORGANISMAL and MOLECULAR Lineages Time 20 lineages One extinction and one speciation event per generation RESULTS: One horizontal transfer event once in
5 generations (I.e., speciation events) Most recent common ancestors are different for organismal and molecular phylogenies RED: organismal lineages (no HGT) BLUE: molecular lineages (with HGT) GRAY: extinct lineages Different coalescence times Long coalescence time for the last two lineages Y chromosome Adam Mitochondrial Eve
Lived approximately 50,000 years ago Lived 166,000-249,000 years ago Thomson, R. et al. (2000) Proc Natl Acad Sci U S A 97, 7360-5 Cann, R.L. et al. (1987) Nature 325, 31-6 Vigilant, L. et al. (1991) Science 253, 1503-7
Underhill, P.A. et al. (2000) Nat Genet 26, 358-61 Albrecht Drer, The Fall of Man, 1504 Adam and Eve never met The same is true for ancestral rRNAs, EF, ATPases! EXTANT LINEAGES FOR THE SIMULATIONS OF 50 LINEAGE Modified from Zhaxybayeva and Gogarten (2004), TIGs 20, 182-187 log (number of surviving lineages) Lineages Through Time Plot 10 simulations of organismal evolution assuming
a constant number of species (200) throughout the simulation; 1 speciation and 1 extinction per time step. (green O) 25 gene histories simulated for each organismal history assuming 1 HGT per 10 speciation events (red x) green: organismal lineages ; red: molecular lineages (with gene transfer) The deviation from the long branches at the base pattern could be due to under sampling an actual radiation due to an invention that was
not transferred following a mass extinction Bacterial 16SrRNA based phylogeny (from P. D. Schloss and J. Handelsman, Microbiology and Molecular Biology Reviews, December 2004.)
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